This invention relates to devices and methods to image or detect useful images at low light levels utilizing passive pixel sensors in an electron bombarded mode using a photocathode for detection or imaging at low light levels.
The copending parent application is directed to the use of active pixel sensors in creating images, particularly of low light level subjects. Active pixel sensor devices comprise a structure or system in which there is gain associated with each pixel in the production of viewable images. Although the use of active pixel sensors enables the production of images from very low light sources or the production of image frames at speeds extending present day capabilities of imaging at low light levels, the use of passive pixel sensors improves upon the sensitivity of certain active pixel sensor systems and thus can produce improved performance in certain low light level conditions. In imaging in which electrons strike the front surface of the pixel, those striking the surface of an active pixel sensor must pass through more transistors to be recognized as compared to the number of transistors encountered in a passive system. This is meaningless if the losses that occur are not important. However, in those systems where each electron is important to the final result and bombardment occurs at the front surface, then a passive system is likely to show less loss as compared to an active one. On the other hand if the amplification of the incoming bombarding electrons is more important to the results than the losses that may be incurred, then an active pixel sensor is to be preferred.
Additionally the use of passive pixel sensors simplifies the making or manufacture of the resulting system. These advantages will become more apparent as this invention is fully discussed hereinafter. For a complete understanding and discussion of the use of active pixel sensor systems, there is incorporated herein by reference the disclosure appearing in Ser. No. 09/356,800, the parent of this application.
Cameras that operate at low light levels have a number of significant applications in diverse areas. These include, among others, photographic, night vision, surveillance, and scientific uses. Modern night vision systems, for example, are rapidly transforming presently used direct view systems to camera based arrangements. These are driven by the continued advances in video display and processing. Video based systems allow remote display and viewing, recording, and image processing including fusion with other imagery such as from a forward looking infra-red sensor. Surveillance applications are also becoming predominately video based where camera size, performance, and low light level sensitivity are often critical. Scientific applications require cameras with good photon sensitivity over a large spectral range and high frame rates. These applications, and others, are driving the need for improved low light level sensors with the capability of a direct video output.
Image sensing devices which incorporate an array of image sensing pixels are commonly used in electronic cameras. Each pixel produces an output signal in response to incident light. The signals are read out, typically one row at a time, to form an image. Cameras in the art have utilized Charge Coupled Devices (CCD) as the image sensor. Image sensors which incorporate an amplifier into each pixel for increased sensitivity are known as active pixel sensors (sometimes referred to herein as APS). Image sensors without an amplifier incorporated in each pixel are known as passive pixel sensors (sometimes referred to herein as PPS). Both APS and PPS imagers belong to the general family of image sensing devices known as CMOS imagers. Active pixel sensors are disclosed, for example in U.S. Pat. No. 5,789,774 issued Aug. 4, 1998 to Merrill; U.S.Pat. No. 5,631,704 issued May 20, 1997 to Dickinson et al; U.S. Pat. No. 5,521,639 issued May 28, 1996 to Tomura et al; U.S. Pat. No. 5,721,425 issued Feb. 24, 1998 to Merrill; U.S. Pat. No. 5,625,210 issued Apr. 29,1997 to Lee et al; U.S. Pat. No. 5,614,744 issued Mar. 25, 1997 to Merrill; and U.S. Pat. No. 5,739,562 issued Apr. 14, 1998 to Ackland et al. Passive pixel sensors are disclosed, for example in U.S. Pat. No. 3,465,293 to Weckler; U.S. Pat. No. 4,631,417 to Brilman; and U.S. Pat. No. 5,345,266 to Denyer. Extensive background on passive and active pixel sensor devices is contained in the paper by Fossum, xe2x80x9cCMOS Image Sensors: Electronic Camera-On-A-Chipxe2x80x9d, IEEE Transactions on Electron Devices, Vol. 44, No. 10, pp. 1689-1698, (1997) and the references therein.
In general, it is desirable to provide cameras which generate high quality images over a wide range of light levels including extremely low light levels such as those encountered under starlight and lower illumination levels. In addition, the camera should have a small physical size and low electrical power requirements, thereby making portable, head-mounted, and other battery-operated applications practical. CMOS image sensor cameras (both APS and PPS) meet the small size and low power requirements, but have poor low light level sensitivity with performance limited to conditions with 0.1 lux (twilight) or higher light levels. Generally APS image sensors have greater sensitivity than PPS image sensors due to the inclusion of amplification in each pixel but amplification, as discussed above requires more transistors per pixel which in turn can result in more photon losses for optical imagers and electron losses for electron sensitive CMOS imagers, which can destroy utility for some applications.
Night vision cameras which operate under extremely low light levels are known in the art. The standard low light level cameras in use today are based on a Generation-III (GaAs photocathode) or Generation-II (multi-alkali photocathode) image intensifier fiber optically coupled to a CCD to form an Image Intensified CCD or ICCD camera. The scene to be imaged is focused by the input lens onto the photocathode faceplate assembly. The impinging light energy liberates photoelectrons from the photocathode to form an electron image. The electron image may, for example, be proximity focused onto the input of the microchannel plate (MCP) electron multiplier, which intensifies the electron image by secondary multiplication while maintaining the geometric integrity of the image. The intensified electron image may also be proximity focused onto a phosphor screen, which converts the electron image back to a visible image, which typically is viewed through a fiber optic output window. A fiber optic taper or transfer lens then transfers this amplified visual image to a standard CCD sensor, which converts the light image into electrons which form a video signal. In these existing prior art ICCD cameras, there are five interfaces at which the image is sampled, and each interface degrades the resolution and adds noise to the signal of the ICCD camera. This image degradation which has heretofore not been avoidable, is a significant disadvantage in systems requiring high quality output. The ICCD sensor tends also to be large and heavy due to the fused fiber optic components. A surveillance system having a Generation-III MCP image intensifier tube is described, for example, in U.S. Pat. No. 5,373,320 issued Dec. 13, 1994 to Johnson et al. A camera attachment described in this patent converts a standard daylight video camera into a day/night video camera.
In addition to image degradation resulting from multiple optical interfaces in the ICCD camera a further disadvantage is that the MCP is a relatively noisy amplifier. This added noise in the gain process further degrades the low light level image quality. The noise characteristics of the MCP can be characterized by the excess noise factor, Kf. Kf is defined as the ratio of the Signal-to-Noise power ratio at the input of the MCP divided by the Signal-to-Noise power ratio at the output of the MCP after amplification. Thus Kf is a measure of the degradation of the image Signal-to-Noise ratio due to the MCP gain process. Typical values for Kf are 4.0 for a Generation-III image intensifier. A low noise, high gain, MCP for use in Generation-III image intensifiers is disclosed in U.S. Pat. No. 5,268,612 issued Dec. 7, 1993 to Aebi et al.
An alternate gain mechanism is achieved by the electron-bombarded semiconductor (sometimes referred to herein as EBS) gain process. In this gain process, gain is achieved by electron multiplication resulting when the high velocity electron beam dissipates its energy in a semiconductor. The dissipated energy creates electron-hole pairs. For the semiconductor silicon one electron-hole pair is created for approximately every 3.6 electron-volt (eV) of incident energy. This is a very low noise gain process with Kf values close to 1. A Kf value of 1 would indicate a gain process with no added noise.
The electron-bombarded semiconductor gain process has been utilized in a focused electron bombarded hybrid photomultiplier tube comprising a photocathode, focusing electrodes and a collection anode comprising a semiconductor diode disposed in a detector body as disclosed in U.S. Pat. No. 5,374,826 issued Dec. 20, 1994 to LaRue et al. and U.S. Pat. No. 5,475,227 issued Dec. 12, 1995 to LaRue. The disclosed hybrid photomultiplier tubes are highly sensitive but do not sense images.
The electron-bombarded semiconductor gain process has been used to address image degradation in the ICCD low light level camera. A back illuminated CCD is used as an anode in proximity focus with the photocathode to form an Electron Bombarded CCD (EBCCD). Photoelectrons from the photocathode are accelerated to and imaged in the back illuminated CCD directly. Gain is achieved by the low noise electron-bombarded semiconductor gain process. The EBCCD eliminates the MCP, phosphor screen, and fiber optics, and as a result both improved image quality and increased sensitivity can be obtained in a smaller sized camera. Significant improvement of the degraded resolution and high noise of the conventional image transfer chain has been realized with the EBCCD. An EBCCD is disclosed in U.S. Pat. No. 4,687,922 issued Aug. 18, 1987 to Lemonier. Extensive background on EBCCDs is contained in the paper by Aebi, et al, xe2x80x9cGallium Arsenide Electron Bombarded CCD Technologyxe2x80x9d, SPIE Vol. 3434, pp. 37-44, (1998) and references cited therein.
Optimum low light level EBCCD performance requires a specialized CCD. The CCD is required to be backside thinned to allow high electron-bombarded semiconductor gain. The CCD cannot be used in a frontside bombarded mode as used in a standard CCD camera as the gate structures would block the photoelectrons from reaching the semiconductor and low electron-bombarded semiconductor gains would be obtained at moderate acceleration voltages. High acceleration voltages required to penetrate the gate structures would cause radiation damage to the CCD and shorten CCD operating life. Also a frame transfer format is required where the CCD has both an imaging region and a store region on the chip. The image and store regions are of approximately the same size. A frame transfer format is required for two reasons. First it is essential that the CCD imaging area have high fill factor (minimum dead area) if possible. The frame transfer CCD architecture satisfies this requirement. The interline transfer CCD architecture would result in substantial dead area (of order 70-80%). Any reduction in active area will result in lost photoelectrons. This is equivalent to a reduction in photocathode quantum efficiency or sensitivity. At the lowest light levels (starlight or overcast starlight), low light level camera performance is dictated by the photon statistics. It is essential that the maximum number of photons be detected by the imager for adequate low light level resolution and performance. Second a frame transfer format allows signal integration to occur during the readout of the store region in addition to any integration period. This allows charge to be integrated almost continuously maximizing the collected signal.
EBCCD cameras have several disadvantages. The frame transfer CCD architecture has the serious disadvantage for the EBCCD application of essentially doubling the size of the required vacuum envelope due to the requirement for image and store regions on the CCD. This requirement also means that the frame transfer CCD chip is more than twice the size of the image area. This substantially increases the cost of the CCD relative to interline transfer CCDs or active or passive pixel sensor chips as fewer chips can be fabricated per silicon wafer. EBCCD based cameras also have the disadvantage of backside illumination of the CCD which necessitates specialized processing to thin the semiconductor and passivate the back surface for high electron-bombarded semiconductor gain. This processing is not standard in the silicon industry and substantially increases the EBCCD manufacturing cost. The EBCCD cameras consume several watts of power due to the CCD clocking requirements and require external electronics for a complete camera. The size of the external camera electronics presents an obstacle to applications that would benefit from miniaturization of the camera. Finally CCDs require specialized semiconductor processing lines which are not compatible with mainstream CMOS semiconductor fabrication technology. This further increases the cost of CCD based cameras.
It is the object of the present invention to further improve upon these various disadvantages in the prior art and provide improved low light level imaging systems and corresponding processes using a passive pixel sensor. This may be achieved by utilizing a passive pixel sensor CMOS imager in an electron bombarded mode in a vacuum envelope with a photocathode sensor. The electron bombarded passive pixel sensor constitutes a complete low light level camera with the addition of a lens, housing, power, and a control interface.
It is accordingly another object of this invention to describe an improved low light level camera which makes use of a passive pixel sensor CMOS imager and direct electron bombardment.
It is yet another object of this invention to describe a novel chip or imaging circuit to facilitate the creation of light-weight structures when this imaging circuit employing passive pixel sensors is used which considerably reduces power requirements and enables improved devices for various and select low light level imaging applications.
Further features and embodiments of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.